The present invention relates to a process according to the preamble of claim 1 for preparing starch esters.
According to such a process a starch-based feedstock is reacted with an organic carboxylic acid anhydride preferably in the presence of a catalyst.
With a view to technical applications, the most important among starch esters are starch acetates and fatty acid esters of starch. Methods for the preparation of starch esters are described in the article by Tessler, M. M. and Bilimers, R. L., Preparation of Starch Esters, in Journal of Environmental Polymer Degradation 4 (1996) 85-89.
Starch acetates can be prepared by allowing (native) starch to react with acetic acid anhydride, i.e. acetanhydride, in the presence of a catalyst. As the catalyst, e.g. a 50% sodium hydroxide is used. The reaction is usually carried out in an aqueous suspension with a solvent such as pyridine or acetic acid anhydride. By varying the amount of the acetic acid anhydride, the amount of the base used as catalyst, and the reaction duration, starch acetates having different degrees of substitution can be prepared. The process described in U.S. Pat. No. 3,795,670, for example, represents the prior art.
The preparation of fatty acid esters of starch is performed, for instance, in the manner described in the following publications: Wolff, I. A., Olds, D. W. and Hilbert, G. E., The Acylation of Corn Starch, Amylose and Amylopectin, J. Amer. Chem. Soc. 73 (1952) 346-349, or Gros, A. T. and Feuge, R. O., Properties of Fatty Acid Esters of Amylose, J. Amer. Oil Chemists"" Soc 39 (1962) 19-24.
The starch esters have usually been separated from the reaction mixture of the esterification reaction by precipitation.
The prior art processes have been hampered by the long reaction time required for the preparation, problems involved in the separation of the esters, and the recirculation of the mother liquor and the recovery of unreacted reactants.
The present invention aims at eliminating the drawbacks related to the prior art and at achieving an entirely novel type of method for the preparation of starch esters.
Esterification is an exothermic reaction and the temperature of the reaction mixture will rise during the reaction until the boiling temperature of the medium is reached. The invention is based on the concept of performing the reaction between the starch-based feedstock and the anhydride at an excess pressure in an essentially anhydrous medium. Even a small rise in temperature (5-10xc2x0 C.) significantly accelerates the reaction and reduces the reaction time by over 50%, typically from about 5 h to 2-3 h. Unexpectedly, no significant degradation of the starch has been observed in spite of the increase in temperature.
The invention is particularly well suited for the preparation of starch acetates whereby acetic acid anhydride is used as the actual reagent in the acetylation of starch, as solvent, acetic acid is sued, and as catalyst, sodium acetate, sodium hydroxide or sulfuric acid is used. Instead of the acetic acid solvent, excess acetanhydride can be used as the environment. Furthermore, acetic acid formed during the reaction can be used as the reaction medium. The starch usually comprises native barley starch but the process is also well suited for the actylation of other starches and starch derivatives. In addition, the process is characterized by only introducing an amount of acetanhydride corresponding to the desired DS, wherein DS is the degree of substitution.
In more detail, the process according to the invention is mainly characterized by what is stated in the characterizing part of claim 1.
The invention provides considerable benefits. Thus, in the context of the process it is easier to control the reaction temperature than in a conventional unpressurized process. The same applies to controlling the viscosity of the reaction. Due to the elevated pressure the reaction time can be shortened because the reaction rate is dependent on the temperature. The regulation of the desired DS is easy because the substitution degree of the final product can be varied by varying the amount of carboxylic acid anhydride; thus, the consumption of the anhydride reagent can be optimized.
According to an advantageous embodiment of the invention, an anhydrous sodium acetate is used as catalyst whereby the amount of water in the reaction medium can be minimized. A controlled hydrolysation of starch is obtained by varying the temperature and the catalyst. The basic concept according to the invention makes a continuous process possible. In the precipitation of the product, several precipitants can be used, such as water and alkanols (e.g. ethanol). The precipitation of the product is made easier because there is less carboxylic acid in the reaction medium; this is because carboxylic acid dissolves particularly esters having a lower degree of substitution.